Okay, so this is a non-standard problem. The game is to play tennis with theorems. Gameplay goes as follows :
(1) A theorem comes your way.
(2) You respond to the theorem with another theorem, which you must relate to the first.
If you play, try to keep the game open, by stating theorems which can easily be related to by others.
I'll begin with a well-known theorem from high-school geometry : the medians of a triangle intersect.